Communication method for cooperative multi-point and wireless device using same

ABSTRACT

A communication method in a wireless communication system, and a wireless device therefore are discussed. The method according to one embodiment includes receiving a first control channel including first scheduling information on a first physical downlink shared channel (PDSCH) to be received in a first subframe; receiving a second control channel including second scheduling information on a second PDSCH to be received in a second subframe; determining whether the first subframe in which the first PDSCH is to be received is overlapped with the second subframe in which the second PDSCH is to be received; and if the first subframe is determined as being overlapped with the second subframe, determining a valid subframe for receiving at least one of the first PDSCH and the second PDSCH.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 14/342,752, filed on Mar. 4, 2014, which is the National Phaseof PCT/KR2012/008568 filed on Oct. 18, 2012, which claims priority under35 U.S.C 119(e) to U.S. Provisional Application No. 61/549,176 filed onOct. 19, 2011, all of which are hereby expressly incorporated byreference into the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a communication method for cooperative multi-point(CoMP) in a wireless communication system, and a wireless device usingthe method.

2. Discussion of the Related Art

3^(rd) generation partnership project (3GPP) long term evolution (LTE)evolved from a universal mobile telecommunications system (UMTS) isintroduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequencydivision multiple access (OFDMA) in a downlink, and uses singlecarrier-frequency division multiple access (SC-FDMA) in an uplink. The3GPP LTE employs multiple input multiple output (MIMO) having up to fourantennas. In recent years, there is an ongoing discussion on 3GPPLTE-advanced (LTE-A) evolved from the 3GPP LTE.

As disclosed in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 8)”, 3GPP LTE/LTE-A may divide the physical channel into adownlink channel, i.e., a physical downlink shared channel (PDSCH) and aphysical downlink control channel (PDCCH), and an uplink channel, i.e.,a physical uplink shared channel (PUSCH) and a physical uplink controlchannel (PUCCH).

A cooperative multi-point (CoMP) technique is one of techniquesintroduced in a next-generation mobile communication system. In general,a user equipment receives a service from one cell. The CoMP allows theuser equipment to receive a service from two cells geographicallyseparated from each other.

In the 3GPP LTE-A, a plurality of serving cells are provided to the userequipment through carrier aggregation. However, there is no ongoingdiscussion on uplink transmission and downlink transmission when thecarrier aggregation is applied to the CoMP.

SUMMARY OF INVENTION

The present invention provides a communication method for cooperativemulti-point (CoMP), and a wireless device using the method.

There is provided a communication method for cooperative multi-point(CoMP) in a wireless communication system. The method may comprise:receiving a first physical downlink shared channel (PDSCH) in a firstsubframe of a first frequency band from a first CoMP cell; and receivinga second PDSCH in a second subframe of a second frequency band in asecond CoMP cell. The first frequency band may overlap partially orentirely with the second frequency band. A first start point at whichthe first PDSCH in the first subframe starts to be scheduled may be thesame as a second start point at which the second PDSCH in the secondsubframe starts to be scheduled.

There is also provided a wireless device for supporting cooperativemulti-point (CoMP) in a wireless communication system. The wirelessdevice may comprise: a radio frequency (RF) unit for transmitting andreceiving a radio signal; and a processor operatively coupled to the RFunit, wherein the processor is configured for: receiving a firstphysical downlink shared channel (PDSCH) in a first subframe of a firstfrequency band from a first CoMP cell; and receiving a second PDSCH in asecond subframe of a second frequency band in a second CoMP cell. Thefirst frequency band may overlap partially or entirely with the secondfrequency band. A first start point at which the first PDSCH in thefirst subframe starts to be scheduled may be the same as a second startpoint at which the second PDSCH in the second subframe starts to bescheduled.

When carrier aggregation is applied to cooperative multi-point (CoMP)and cells use the same frequency band, inter-cell interference can bemitigated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a structure of a downlink radio frame in 3rd generationpartnership project (3GPP) long term evolution (LTE).

FIG. 2 shows an example of multiple carriers.

FIG. 3 shows an example of cooperative multi-point (CoMP) transmissionusing carrier aggregation (CA).

FIG. 4 shows a cooperative multi-point (CoMP) operation according to anembodiment of the present invention.

FIG. 5 shows an example in which a different control format indicator(CFI) is given to each cell.

FIG. 6 is a block diagram of a wireless communication system accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A wireless device may be fixed or mobile, and may be referred to asanother terminology, such as a user equipment (UE), a mobile station(MS), a user terminal (UT), a subscriber station (SS), a mobile terminal(MT), etc. A base station (BS) is generally a fixed station thatcommunicates with the wireless device, and may be referred to as anotherterminology, such as an evolved-NodeB (eNB), a base transceiver system(BTS), an access point, etc.

It is described hereinafter that the present invention is applied basedon 3rd generation partnership project (3GPP) long term evolution (LTE)or 3GPP LTE-advanced (LTE-A). This is for exemplary purposes only, andthe present invention is also applicable to various wirelesscommunication systems. In the following description, LTE and/or LTE-Aare collectively referred to as LTE.

FIG. 1 shows a structure of a downlink radio frame in 3GPP LTE. Thesection 6 of 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 8)” may be incorporated herein by reference.

A radio frame includes 10 subframes indexed with 0 to 9. One subframeincludes 2 consecutive slots. A time required for transmitting onesubframe is defined as a transmission time interval (TTI). For example,one subframe may have a length of 1 millisecond (ms), and one slot mayhave a length of 0.5 ms.

One slot may include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlink(DL), the OFDM symbol is only for expressing one symbol period in thetime domain, and there is no limitation in a multiple access scheme orterminologies. For example, the OFDM symbol may also be referred to asanother terminology such as a single carrier frequency division multipleaccess (SC-FDMA) symbol, a symbol period, etc.

Although it is described that one slot includes 7 OFDM symbols forexample, the number of OFDM symbols included in one slot may varydepending on a length of a cyclic prefix (CP). According to 3GPP TS36.211 V8.7.0, in case of a normal CP, one slot includes 7 OFDM symbols,and in case of an extended CP, one slot includes 6 OFDM symbols.

A resource block (RB) is a resource allocation unit, and includes aplurality of subcarriers in one slot. For example, if one slot includes7 OFDM symbols in a time domain and the RB includes 12 subcarriers in afrequency domain, one RB can include 7×12 resource elements (REs).

A DL subframe is divided into a control region and a data region in thetime domain. The control region includes up to first four OFDM symbolsof a first slot in the subframe. However, the number of OFDM symbolsincluded in the control region may vary. A physical downlink controlchannel (PDCCH) and other control channels are allocated to the controlregion, and a physical downlink shared channel (PDSCH) is allocated tothe data region.

As disclosed in 3GPP TS 36.211 V8.7.0, the 3GPP LTE classifies aphysical channel into a data channel and a control channel. Examples ofthe data channel include a physical downlink shared channel (PDSCH) anda physical uplink shared channel (PUSCH). Examples of the controlchannel include a physical downlink control channel (PDCCH), a physicalcontrol format indicator channel (PCFICH), a physical hybrid-ARQindicator channel (PHICH), and a physical uplink control channel(PUCCH).

The PCFICH transmitted in a first OFDM symbol of the subframe carries acontrol format indicator (CFI) regarding the number of OFDM symbols(i.e., a size of the control region) used for transmission of controlchannels in the subframe. The wireless device first receives the CFI onthe PCFICH, and thereafter monitors the PDCCH.

Unlike the PDCCH, the PCFICH does not use blind decoding, and istransmitted by using a fixed PCFICH resource of the subframe.

The PHICH carries a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal for an uplink hybridautomatic repeat request (HARD). The ACK/NACK signal for uplink (UL)data on a PUSCH transmitted by the wireless device is transmitted on thePHICH.

A physical broadcast channel (PBCH) is transmitted in first four OFDMsymbols in a second slot of a first subframe of a radio frame. The PBCHcarries system information necessary for communication between thewireless device and a BS. The system information transmitted through thePBCH is referred to as a master information block (MIB). In comparisonthereto, system information transmitted on the PDCCH is referred to as asystem information block (SIB).

Control information transmitted through the PDCCH is referred to asdownlink control information (DCI). The DCI may include resourceallocation of the PDSCH (this is referred to as a downlink (DL) grant),resource allocation of a PUSCH (this is referred to as an uplink (UL)grant), a set of transmit power control commands for individual UEs inany UE group, and/or activation of a voice over Internet protocol(VoIP).

The 3GPP LTE/LTE-A uses blind decoding for PDCCH detection. The blinddecoding is a scheme in which a desired identifier is de-masked from acyclic redundancy check (CRC) of a received PDCCH (referred to as acandidate PDCCH) to determine whether the PDCCH is its own controlchannel by performing CRC error checking.

The BS determines a PDCCH format according to DCI to be transmitted tothe UE, attaches a CRC to the DCI, and masks a unique identifier(referred to as a radio network temporary identifier (RNTI)) to the CRCaccording to an owner or usage of the PDCCH.

A control region in a subframe includes a plurality of control channelelements (CCEs). The CCE is a logical allocation unit used to providethe PDCCH with a coding rate depending on a radio channel state, andcorresponds to a plurality of resource element groups (REGs). The REGincludes a plurality of resource elements. According to an associationrelation of the number of CCEs and the coding rate provided by the CCEs,a PDCCH format and the number of bits of an available PDCCH aredetermined.

One REG includes 4 REs. One CCE includes 9 REGs. The number of CCEs usedto configure one PDCCH may be selected from a set {1, 2, 4, 8}. Eachelement of the set {1, 2, 4, 8} is referred to as a CCE aggregationlevel.

The BS determines the number of CCEs used in transmission of the PDCCHaccording to a channel state. For example, a wireless device having agood DL channel state can use one CCE in PDCCH transmission. A wirelessdevice having a poor DL channel state can use 8 CCEs in PDCCHtransmission.

A control channel consisting of one or more CCEs performs interleavingon an REG basis, and is mapped to a physical resource after performingcyclic shift based on a cell identifier (ID).

According to 3GPP TS 36.211 V8.7.0, the uplink channel includes a PUSCH,a PUCCH, a sounding reference signal (SRS), and a physical random accesschannel (PRACH).

The PUCCH supports multiple formats. A PUCCH having a different numberof bits per subframe can be used according to a modulation scheme whichis dependent on the PUCCH format. The PUCCH format 1 is used fortransmission of a scheduling request (SR). The PUCCH formats 1a/1b areused for transmission of an ACK/NACK signal. The PUCCH format 2 is usedfor transmission of a CQI. The PUCCH formats 2a/2b are used forsimultaneous transmission of the CQI and the ACK/NACK signal. When onlythe ACK/NACK signal is transmitted in a subframe, the PUCCH formats1a/1b are used. When the SR is transmitted alone, the PUCCH format 1 isused. When the SR and the ACK/NACK are simultaneously transmitted, thePUCCH format 1 is used, and in this transmission, the ACK/NACK signal ismodulated by using a resource allocated to the SR.

Now, a multiple-carrier system will be described.

Spectrum aggregation (or bandwidth aggregation, also referred to ascarrier aggregation) is that a network supports a plurality of componentcarriers (CCs). For example, if 5 CCs are assigned as a granularity of acarrier unit having a bandwidth of 20 MHz, a bandwidth of up to 100 MHzcan be supported.

One DL CC or a pair of a UL CC and a DL CC can be mapped to one cell.Therefore, when a UE communicates with a BS through a plurality of CCs,it can be said that the UE receives a service from a plurality ofserving cells.

FIG. 2 shows an example of multiple carriers.

Although three DL CCs and three UL CCs are shown herein, the number ofDL CCs and the number of UL CCs are not limited thereto. A PDCCH and aPDSCH are independently transmitted in each DL CC. A PUCCH and a PUSCHare independently transmitted in each UL CC. Since three DL CC-UL CCpairs are defined, it can be said that a wireless device receives aservice from three serving cells.

The wireless device can monitor the PDCCH in a plurality of DL CCs, andcan receive a DL transport block simultaneously via the plurality of DLCCs. The wireless device can transmit a plurality of UL transport blockssimultaneously via a plurality of UL CCs.

It is assumed that a pair of a DL CC #1 and a UL CC #1 is a 1^(st)serving cell, a pair of a DL CC #2 and a UL CC #2 is a 2^(nd) servingcell, and a DL CC #3 is a 3^(rd) serving cell. Each serving cell can beidentified by using a cell index (CI). The CI may be cell-specific orUE-specific. Herein, CI=0, 1, 2 are assigned to the 1^(st) to 3^(rd)serving cells for example.

The serving cell can be classified into a primary cell and a secondarycell. The primary cell operates at a primary frequency, and is a celldesignated as the primary cell when the wireless device performs aninitial network entry process or starts a network re-entry process orperforms a handover process. The primary cell is also called a referencecell. The secondary cell operates at a secondary frequency. Thesecondary cell can be configured after an RRC connection is established,and can be used to provide an additional radio resource.

At least one primary cell is configured always. The secondary cell canbe added/modified/released by using higher-layer signaling (e.g., RRCmessages).

The CI of the primary cell may be fixed. For example, a lowest CI can bedesignated as a CI of the primary cell. It is assumed hereinafter thatthe CI of the primary cell is 0 and a CI of the secondary cell isallocated sequentially starting from 1.

The wireless device can monitor a PDCCH through a plurality of servingcells. However, even if there are N serving cells, the BS can beconfigured to monitor the PDCCH for M (M≦N) serving cells. In addition,the BS can be configured to preferentially monitor the PDCCH for L(L≦M≦N) serving cells.

Meanwhile, a technique which implements both carrier aggregation (CA)and cooperative multi-point (CoMP) is proposed. The CA is to support aplurality of cells by one BS, whereas the CoMP using the CA is tosupport a plurality of cells by a plurality of BSs.

FIG. 3 shows an example of CoMP transmission using CA.

It is assumed that a serving BS corresponds to a primary cell, acoordinating BS corresponds to a secondary cell, and the primary celland the secondary cell use the same regions in a frequency domain.

A cell which causes an interference to the primary cell may be allocatedas the secondary cell, and then CoMP transmission may be implemented byusing well-known various schemes such as joint transmission, dynamicpoint selection (DPS), coordinated beam forming, coordinated scheduling,etc.

Hereinafter, it is proposed a method for operating a CoMP operation insuch a manner that, from a perspective of a wireless device, a pluralityof cells participating in the CoMP operation is configured as if CA isapplied to the cells.

For convenience of explanation, the following terminologies are defined.

CA set: It is a set of cells (to which CA is applied) which can beconfigured to the wireless device.

CA cell: It is a cell belonging to the CA set.

PCell: It is one cell among cells belonging to the CA set. Morespecifically, a cell to which an RRC connection is initially establishedby the wireless device may be designated as the PCell. The wirelessdevice may receive a physical channel (e.g., PBCH) for obtainingimportant system information through the PCell. The wireless device maytransmit a PUCCH through the PCell.

SCell: It is a cell which is not the PCell. It may be activated orinactivated by the PCell.

CoMP set: It is a set of cells to which a CoMP operation is appliedamong cells belonging to the CA set. It may include a cell participatingin signaling or transmission/reception for CoMP or a cell which is acandidate for participating therein. The CoMP set may be identical tothe CA set, or may be a subset of the CA set.

CoMP cell: It is a cell belonging to the CoMP set.

CoMP PCell: It is one cell among cells belonging to the CoMP set. A CoMPPCell may be identical to the PCell. Alternatively, in the CoMP PCell,RRC signaling may be configured separately from the PCell. The CoMPPCell may include a cell for transmitting a PDCCH which schedulesPDSCH/PUSCH transmission for the CoMP cells.

CoMP SCell: It is a cell which is not a CoMP PCell among cells belongingto the CoMP set.

Hereinafter, although it is assumed a case where CoMP cells use anoverlapping frequency band (or CC), it is also possible that the CoMPcells use different frequency bands.

Although one CoMP set and one CA set are described for example, thenumber of CoMP sets and the number of CA sets are for exemplary purposesonly.

The CoMP set and/or the CA set may be a set of cells allocated to awireless device by a BS. Information regarding the CoMP set and/or theCA set may be transmitted through RRC/MAC signaling by the BS to thewireless device.

The CoMP set may indicate a set of cells classified for a specificpurpose also in cells belonging to the CA set.

The CoMP technique proposed hereinafter may be applied restrictively toa specific time duration (e.g., a subframe unit).

First, the proposed UL/DL scheduling will be described.

FIG. 4 shows a CoMP operation according to an embodiment of the presentinvention.

In the conventional CA operation, a DL channel (e.g., a PDSCH) may betransmitted through different CA cells in one time point (e.g., the samesubframe) with respect to one wireless device. However, when it isapplied to PDSCH transmission for a CoMP set with respect to onewireless device, if different CoMP cells transmit a PDSCH through anoverlapping frequency band in the same time point, DL data receptioncapability may deteriorate due to interference. Therefore, the presentinvention proposes the following methods.

A serving BS and/or a coordinating BS may neither schedule nor transmita PDSCH for two or more CoMP cells in the same subframe. The wirelessdevice may not expect to receive a PDCCH for scheduling a PDSCH from thetwo or more CoMP cells in the same subframe, or may not performmonitoring.

If the wireless device receives a PDCCH for scheduling two PDSCHs withrespect to two or more CoMP cells in the same subframe, PDSCH schedulingbased on the PDCCH may be ignored. Alternatively, the wireless devicemay receive only selected PDSCH scheduling attempt, and may ignore theremaining PDSCH scheduling attempts. The selected PDSCH scheduling mayinclude PDSCH scheduling for the CoMP PCell.

The wireless device may preferentially detect PDSCH scheduling for theCoMP PCell. Upon detecting the PDSCH scheduling for the CoMP PCell, thewireless device may not monitor the PDCCH for the PDSCH scheduling forthe CoMP SCell.

PDSCH transmission which is not scheduled to a PDCCH (e.g., a PDSCH forsemi-persistent scheduling (SPS), this is called as a PDSCH without aPDCCH) may be configured for one CoMP cell, and a PDSCH may be scheduledto another CoMP cell. The wireless device may discard PDSCH receptionwithout a PDCCH, and may receive only a PDSCH scheduled to the PDCCH.Alternatively, the wireless device may preferentially receive the PDSCHwithout the PDCCH.

If a PDCCH for scheduling a PDSCH is detected for a CoMP cell to whichthe PDSCH without the PDCCH is assigned, the wireless device may discardtransmission of the PDSCH without the PDCCH, and may receive the PDSCHscheduled to the detected PDCCH.

It may be configured such that only a CoMP PCell schedules a PDSCH ofall CoMP cells. That is, it is a case where a PDCCH for DL scheduling istransmitted in the CoMP PCell. All CoMP cells may share the same searchspace (i.e., a UE specific search space or a common search space) forPDSCH scheduling and may monitor the PDCCH in the search space. In orderto decrease the number of PDCCH blind detections, the sharing of thesearch space for a plurality of CoMP cells may apply only for a DCIhaving the same size. To share the search space, the cells may beconfigured to have the same transmission mode or the same bandwidth.

To share the search space, a bit may be added to the DCI for some or allof CoMP cells to have the same DCI size, or a unit amount of resourcefor scheduling a PDSCH or the total resource amount capable ofscheduling the PDSCH may be regulated.

Although it is related to the DL scheduling, UL scheduling may also beperformed in a similar manner.

In the conventional CA operation, a UL channel (e.g., a PUSCH) may betransmitted through different CA cells in one time point (e.g., the samesubframe) with respect to one wireless device. However, when it isapplied to PUSCH transmission for a CoMP set with respect to onewireless device, if different CoMP cells transmit a PUSCH through anoverlapping frequency band in the same time point, UL data receptioncapability may deteriorate due to interference. Therefore, the presentinvention proposes the following methods.

A serving BS and/or a coordinating BS may neither schedule nor transmita PUSCH for two or more CoMP cells in the same subframe. The wirelessdevice may not expect to receive a PDCCH for scheduling a PUSCH from thetwo or more CoMP cells in the same subframe, or may not performmonitoring.

If the wireless device receives a PDCCH for scheduling two PUSCHs withrespect to two or more CoMP cells in the same subframe, PDSCH schedulingbased on the PUCCH may be ignored. Alternatively, the wireless devicemay receive only selected PUSCH scheduling attempt, and may ignore theremaining PDSCH scheduling attempts. The selected PDSCH scheduling mayinclude PUSCH scheduling for the CoMP PCell.

The wireless device may preferentially detect PUSCH scheduling for theCoMP PCell. Upon detecting the PUSCH scheduling for the CoMP PCell, thewireless device may not monitor the PDCCH for the PUSCH scheduling forthe CoMP SCell.

PUSCH transmission which is not scheduled to a PDCCH (e.g., a PUSCH forsemi-persistent scheduling (SPS), this is called as a PUSCH without aPDCCH) may be configured for one CoMP cell, and a PUSCH may be scheduledto another CoMP cell. The wireless device may discard PUSCH receptionwithout a PDCCH, and may receive only a PUSCH scheduled to the PDCCH.Alternatively, the wireless device may preferentially receive the PUSCHwithout the PDCCH.

If a PDCCH for scheduling a PUSCH is detected for a CoMP cell to whichthe PUSCH without the PDCCH is assigned, the wireless device may discardtransmission of the PUSCH without the PDCCH, and may receive the PUSCHscheduled to the detected PDCCH.

It may be configured such that only a CoMP PCell schedules a PUSCH ofall CoMP cells. That is, it is a case where a PDCCH for DL scheduling istransmitted in the CoMP PCell. All CoMP cells may share the same searchspace (i.e., a UE specific search space or a common search space) forPUSCH scheduling and may monitor the PDCCH in the search space. In orderto decrease the number of PDCCH blind detections, the sharing of thesearch space for a plurality of CoMP cells may apply only for a DCIhaving the same size. To share the search space, the cells may beconfigured to have the same transmission mode or the same bandwidth.

To share the search space, a bit may be added to the DCI for some or allof CoMP cells to have the same DCI size, or a unit amount of resourcefor scheduling a PUSCH or the total resource amount capable ofscheduling the PUSCH may be regulated.

In the existing 3GPP LTE, the PCFICH transmitted in a first OFDM symbolof the subframe carries a control format indicator (CFI) regarding thenumber of OFDM symbols (i.e., a size of the control region) used fortransmission of control channels in the subframe. That is, the CFIindicates the size of the control region, the size of the data region,and a start point at which PDSCH transmission starts. A wireless devicefirst receives the CFI on the PCFICH, and thereafter monitors the PDCCH.

When the PDCCH is transmitted only in the PCell, the wireless deviceacquires a CFI in every subframe through a PCFICH of the PCell. The CFIin the SCell may be given semi-statically not through the PCFICH butthrough RRC signaling.

FIG. 5 shows an example in which a different CFI is given to each cell.

However, when a plurality of cells within a CoMP set simultaneouslytransmit a PDSCH to one wireless device and the wireless device combinesa plurality of PDSCHs by using joint coding for example, receptioncapability may deteriorate if a transmission start point of the PDSCHdiffers from one cell to another. Therefore, the start point of thePDSCH transmitted to one wireless device in the same subframe within theCoMP set needs to be adjusted identically. Therefore, the presentinvention proposes the following methods.

In a first embodiment, a CFI value in each subframe of cells in a CoMPset may depend on a CFI in a specific cell (e.g., CoMP PCell). The CFIof the specific cell may be transmitted to the wireless device through aPCFICH or extra signaling.

In a second embodiment, all of the cells within the CoMP set conform toa CFI determined through RRC/MAC signaling. Even if the CoMP PCell orthe PCell exists in the CoMP set and their CFIs are designated, all ofthe cells in the CoMP set may conform to the CFI determined throughRRC/MAC signaling.

A CoMP operation may apply differently according to a subframe. Assumethat a subframe to which the CoMP operation is applied is called a CoMPsubframe, and a subframe to which the CoMP operation is not applied iscalled a non-CoMP subframe. For example, a PDSCH to which the CoMPoperation is applied may be transmitted in the CoMP subframe. Only theCoMP subframe may conform to the CFI determined through the RRC/MACsignaling. The RRC/MAC signaling may be applied independently for eachCoMP set.

In a third embodiment, a common CFI may be transmitted through a PDCCHtransmitted in each cell in the CoMP set. The common CFI refers to a CFIcommonly applied to cells in the CoMP set. If a CFI having a differentvalue is received from the cells in the CoMP set, the CFI may beignored.

In a fourth embodiment, a CFI of each cell in a CoMP set is set to amaximum possible CFI value. The maximum CFI value may be given for eachsubframe.

In a fifth embodiment, a CFI of each cell in a CoMP set may bedesignated as a maximum value among CFI values acquired by a PCFICHtransmitted from a cell in which a PDCCH in the CoMP set is received.

In a sixth embodiment, if a PDCCH for scheduling a plurality of CoMPcells in one CoMP cell is received in a specific subframe (or a specificduration), a CFI defined in one CoMP cell may be applied to the CoMPset.

As described above, when a PDSCH is transmitted only in one cell at onetime point (or the same subframe) among cells belonging to a CoMP set,ACK/NACK for the PDSCH is not necessarily designed by considering allcells. Therefore, it is proposed to configure ACK/NACK information.

Hereinafter, it is assumed that a 1-bit ACK/NACK corresponds to onetransport block (or codeword) transmitted on the PDSCH. This is forexemplary purposes only, and thus a plurality of transport blocks may betransmitted on the PDSCH in one cell, and the 1-bit ACK/NACK may be avalue obtained by performing an AND operation on the plurality oftransport blocks.

As to the cells in the CoMP set, the number of bits of an ACK/NACKpayload transmitted in one subframe by the wireless device may be setaccording to ACK/NACK for one cell. As to cells not belonging to theCoMP set, ACK/NACK that can be transmitted to the maximum extentpossible for each cell is set to the number of bits of the ACK/NACKpayload. For example, if the CoMP set includes 3 cells and each celltransmits one transport block, the ACK/NACK payload transmitted in onesubframe is defined as 1 bit. If the number of cells not belonging tothe CoMP set is 3 and each cell transmits one transport block, theACK/NACK payload transmitted in one subframe is defined as 3 bits. Itcan be generalized such that if a CA set has M cells and if N(M>N) cellsbelong to the same CoMP set, then the ACK/NACK payload is (M−N+1) bits.

If the maximum number of transmissible transport blocks differs for eachcell belonging to the CoMP set, the number of bits of the ACK/NACKpayload may be defined according to a cell having the maximum number oftransport blocks. For example, if the maximum number of transport blocksof a first CoMP cell is 1 and the maximum number of transport blocks ofa second CoMP cell is 2, the number of bits of the ACK/NACK payload is2.

When ACK/NACK for the CoMP cell and ACK/NACK for a non-CoMP cell set aretransmitted together, the ACK/NACK payload may be defined as follows.

In a first embodiment, CoMP cells may be configured to have consecutivecell indices so that ACK/NACK for the CoMP cell is a consecutive bit inthe ACK/NACK payload. Therefore, the ACK/NACK bit for the CoMP cells andthe ACK/NACK bit for the non-CoMP cell must not be mixed.

The cell index of the CoMP cell may be defined in an ascending ordescending order of an index among cells belonging to the CA set.Alternatively, it is also possible to configure such that a PCellbelongs to a CoMP set, a cell index of the PCell is set to 0, and cellindices of the remaining CoMP cells are allocated consecutively.

In a second embodiment, a position of ACK/NACK for a CoMP cell may befixed within an ACK/NACK payload. For example, the ACK/NACK for the CoMPcell is configured as a least significant bit (LSB) or a mostsignificant bit (MSB) in the ACK/NACK payload. Alternatively, theposition of ACK/NACK for the CoMP cell may be a position next toACK/NACK of a PCell in the ACK/NACK payload. If the PCell belongs to theCoMP set, the position of ACK/NACK for the CoMP set may be the positionof ACK/NACK for the PCell. For example, if a cell index of the PCell is0, ACK/NACK for the CoMP cell may be allocated starting from a first bitof the ACK/NACK payload.

In a third embodiment, the position of ACK/NACK for the CoMP cell may beset according to the position of ACK/NACK for a specific cell amongcells belonging to the CoMP set. The specific cell may be a CoMP PCellor a cell having a greatest or smallest cell index among cells belongingto the CoMP set. Therefore, ACK/NACK for the CoMP set may be arrangedaccording to the specific cell in the ACK/NACK payload for all CA sets.

An ACK/NACK payload including ACK/NACK for a plurality of cells may betransmitted through a PUCCH format 3 or a PUSCH. The ACK/NACK payloadmay be transmitted by being expressed or compressed with ACK/NACKchannel selection, ACK/NACK bundling, channel coding, modulation, and acombination thereof.

When a wireless device supports a plurality of CA cells, if a PUSCH isscheduled in a subframe which transmits uplink control information (UCI)such as ACK/NACK, channel state information (CSI), etc., the UCI may bepiggybacked on the PUSCH. The ‘piggyback’ is that the UCI is transmittedby being multiplexed to UL traffic to be transmitted on the PUSCH. WhenACK/NACK for a plurality of cells is piggybacked on the PUSCH, the UCImay be piggybacked preferentially on a PCell.

When a PUSCH for a plurality of CoMP cells is transmitted in a subframein which the wireless device transmits the UCI, the UCI may bepiggybacked preferentially on the PUSCH transmitted through a CoMP PCellrather than a CoMP SCell. If the PCell does not belong to the CoMP setbut there is PUSCH transmission through the PCell, the wireless devicemay piggyback the UCI on the PUSCH transmitted to the PCell, or maypiggyback the UCI through the PUSCH transmitted to the CoMP PCellirrespective of PUSCH transmission of the PCell.

When the wireless device supports a plurality of CA cells and feeds backCSI for the plurality of CA cells, CSI for one or some CA cells may beselectively transmitted, and CSI feedback for the remaining CA cells maybe discarded. In this case, the CSI for the PCell may be transmittedpreferentially over other cells.

It is proposed to preferentially transmit the CSI for the CoMP PCellrather than the CSI of the CoMP SCell when the wireless device transmitsthe CSI for the plurality of CoMP cells. CSI transmission for the CoMPSCell may be discarded preferentially over CSI transmission for the CoMPPCell. For a priority for the CoMP PCell, a different reference (e.g.,CSI content, period, etc., of each CoMP cell) may be first applied, andthen may be applied as a next step after selecting a cell for discardingCSI transmission.

The CSI for the CoMP PCell may be transmitted preferentially over anSCell not belonging to the CoMP set or all cells not belonging to theCoMP set.

In order to implement CSI prioritization for the CoMP set, the CoMPPCell within the CoMP set may be assigned a smaller cell index than theCoMP SCell. For example, a smallest cell index may be assigned to theCoMP PCell, and then a priority may be assigned according to a cellindex.

UL transmission timing for each of a plurality of CA cells of thewireless device may be regulated independently for each timing advancegrout (TAG). The plurality of CA cells belonging to one TAG may sharethe same UL transmission timing. The same timing advance command (TAC)is applied to cells belonging to the same TAG, and such cells share atime alignment timer.

For a CoMP operation, when UL transmission is constant irrespective of atarget cell, an implementation can be simply achieved from a perspectiveof BS reception or UE transmission. Therefore, cells in the same CoMPset may belong to the same TAG. That is, cells in the same CoMP set mayshare the same UL transmission timing.

To regulate UL transmission timing, a random access procedure may beperformed. A random access preamble may be transmitted through a CoMPPCell. In addition, the wireless device may receive a PDCCH fortriggering transmission of the random access preamble in the CoMP PCell.

UL transmission timing may be regulated by using the random accessprocedure only for the CoMP PCell, and the CoMP SCell may determine ULtransmission timing by using a TAC in the CoMP PCell.

As described above, when a plurality of cells belonging to a CoMP setsimultaneously transmit a UL channel (e.g., PUCCH, PUCCH, SRS) in onesubframe, successful reception of a BS may be difficult due tointerference. Therefore, UL channel transmission is proposed as follows.Hereinafter, when it is said that SRS/PUSCH/PUCCH is transmitted in aCoMP cell, it may imply that SRS/PUSCH/PUCCH is transmitted according toparameters configured in the cell.

The wireless device may transmit the SRS only in the selected CoMP cellin one subframe among CoMP cells included in the CoMP set. In SRStransmission, the CoMP PCell may have a higher priority than the CoMPSCell. The SRS transmission may not be allowed in the CA cell notbelonging to the CoMP set.

When the wireless device transmits the SRS in one CoMP cell included inthe CoMP set, the PUSCH or PUCCH of the remaining CoMP cell may not betransmitted in an OFDM symbol in which the SRS is transmitted or maydiscard PUSCH/PUCCH transmission. SRS/PUSCH/PUCCH transmission may beallowed in the CA cell not belonging to the CoMP set.

FIG. 6 is a block diagram of a wireless communication system accordingto an embodiment of the present invention.

A BS 50 includes a processor 51, a memory 52, and a radio frequency (RF)unit 53. The memory 52 is coupled to the processor 51, and stores avariety of information for driving the processor 51. The RF unit 53 iscoupled to the processor 51, and transmits and/or receives a radiosignal. The processor 51 implements the proposed functions, procedures,and/or methods. In the aforementioned embodiment, an operation of the BSmay be implemented by the processor 51.

A wireless device 60 includes a processor 61, a memory 62, and an RFunit 63. The memory 62 is coupled to the processor 61, and stores avariety of information for driving the processor 61. The RF unit 63 iscoupled to the processor 61, and transmits and/or receives a radiosignal. The processor 61 implements the proposed functions, procedures,and/or methods. In the aforementioned embodiment, an operation of thewireless device may be implemented by the processor 61.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The RF unit may include a base-bandcircuit for processing a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory and may be performed by the processor. The memory may be locatedinside or outside the processor, and may be coupled to the processor byusing various well-known means.

Although the aforementioned exemplary system has been described on thebasis of a flowchart in which steps or blocks are listed in sequence,the steps of the present invention are not limited to a certain order.Therefore, a certain step may be performed in a different step or in adifferent order or concurrently with respect to that described above.Further, it will be understood by those ordinary skilled in the art thatthe steps of the flowcharts are not exclusive. Rather, another step maybe included therein or one or more steps may be deleted within the scopeof the present invention.

What is claimed is:
 1. A communication method in a wirelesscommunication system, the method comprising: receiving a first controlchannel including first scheduling information on a first physicaldownlink shared channel (PDSCH) to be received in a first subframe;receiving a second control channel including second schedulinginformation on a second PDSCH to be received in a second subframe;determining whether the first subframe in which the first PDSCH is to bereceived is overlapped with the second subframe in which the secondPDSCH is to be received; and if the first subframe is determined asbeing overlapped with the second subframe, determining a valid subframefor receiving at least one of the first PDSCH and the second PDSCH. 2.The method of claim 1, wherein the first and second subframes include aplurality of orthogonal frequency division multiplexing (OFDM) symbols.3. The method of claim 2, wherein: the first PDSCH is to be received ona first start point in the first subframe, and the second PDSCH is to bereceived on a second start point in the second subframe.
 4. The methodof claim 3, wherein the first and second start points are indicated byone control format indicator (CFI).
 5. The method of claim 4, wherein:the first PDSCH is to be received from a first cell, and the secondPDSCH is to be received from a second cell.
 6. A wireless device forsupporting cooperative multi-point (CoMP) in a wireless communicationsystem, the wireless device comprising: a radio frequency (RF) unitconfigured to transmit and receive a radio signal; and a processoroperatively coupled to the RF unit, wherein the processor is configuredto: receive a first control channel including first schedulinginformation on a first physical downlink shared channel (PDSCH) to bereceived in a first subframe, receive a second control channel includingsecond scheduling information on a second PDSCH to be received in asecond subframe, determine whether the first subframe in which the firstPDSCH is to be received is overlapped with the second subframe in whichthe second PDSCH is to be received, and if the first subframe isdetermined as being overlapped with the second subframe, determine avalid subframe for receiving at least one of the first PDSCH and thesecond PDSCH.
 7. The wireless device of claim 6, wherein the first andsecond subframes include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols.
 8. The wireless device of claim 7, wherein:the first PDSCH is to be received on a first start point in the firstsubframe, and the second PDSCH is to be received on a second start pointin the second subframe.
 9. The wireless device of claim 8, wherein thefirst and second start points are indicated by one control formatindicator (CFI).
 10. The wireless device of claim 9, wherein: the firstPDSCH is to be received from a first cell, and the second PDSCH is to bereceived from a second cell.